The present invention relates to an electric double layer capacitor and a method of forming the same, and more particularly to an improvement in structure of a collector and a polarization electrode of an electric double layer capacitor which has a large capacity for various purposes, for example, of a semiconductor memory back-up.
FIG. 1 is a schematic cross sectional elevation view illustrative of a structure of a basic cell or a capacitor element of a conventional electric double layer capacitor. A pair of polarization electrodes 10 is provided. A separator 12 is also provided which separates the paired polarization electrodes 10. A pair of collectors 11 is provided, so that the paired collectors 11 sandwich the paired polarization electrodes 10. The sandwiched structure comprising the paired polarization electrodes 10 and the separator 12 has opposite ends which are spaced apart from each other in a direction parallel to interfaces between the separator 12 and the paired polarization electrodes 10. The opposite ends of the sandwiched structure are sealed with a gasket 13. Each of the paired polarization electrodes 10 is permeated with an electrolyte to form an electric double layer structure. Further, a pair of terminal plates 14 is provided, wherein the paired terminal plates 14 are in contact with the paired collectors. 11.
It is necessary that the polarization electrodes 10 are electrically conductive and stable to the used electrolyte as well as have a large surface area. Powders or fibers of activated carbons such as coconut shell based activated carbon, or one of those activated carbons solidified by a binder such as polyetrafluoroethylene are available for material of the polarization electrodes 10. The polarization electrodes 10 are permeated with an electrolyte therein. An available electrolyte of solution type is, for example, a sulfuric acid and potassium hydroxide. An available electrolyte of organic solvent type is, for example, quaternary ammonium salt. If the solution type electrolyte is used, then the collector 11 may be made of an organic material such as a rubber provided with a conductivity, for example, carbon powders. If the organic solvent type electrolyte is used, then the collector 11 may comprise a metal film. In order to reduce an internal resistance of the polarization electrodes 10 themselves and also reduce sa contact resistance between the polarization electrode 10 and the collector 11, a thermal fusion splicing between the polarization electrode 10 and the collector 11 is carried out so that projections on a surface of the polarization electrode 10 partially enter into the surface of the collector 11, if the collector 11 may be made of the organic material such as the rubber with the conductive powders. The separator 12 comprises a porous membrane with an ion-permeability but without electrical conductivity, for example, a polypropylene porous membrane. The gasket 13 may comprise an insulation rubber for maintaining a shape of the basic cell and preventing any leakage of the electrolyte as well as preventing a short circuit due to contact of the collectors 11 to a case. The terminal plates 14 are pressured toward the collectors 11 so as to reduce a contact resistance between the terminal plates 14 and the collectors 11.
A withstand voltage of the basic cell depends upon the kind of the electrolyte. If, for example, the solution type electrolyte is used, then the withstand voltage is about 1.0V. If the organic solvent type electrolyte is used, then the withstand voltage is in the range of about 2.0 V to 3.0V. Therefore, in order to obtain the required withstand voltage, a plurality of the basic cells are laminated in series connection.
In the past, the electric double layer capacitor has been used for the purpose of applying a relatively small current, such as a back-up of a memory. In recent years, however, the electric double layer capacitor has been used for not only the above purpose but also another purpose of applying a large current in other fields of automobile, and electronic components which need relatively large currents. In order to apply a relatively large current, it is important to reduce the thicknesses of the polarization electrode and the collector so as to reduce an equivalent series resistance (ESR) of the basic cell. Upon the requirement for size-down of the electronic component, it is also required to reduce the thickness of the electric double layer capacitor.
The conventional electric double layer capacitor has the following problems. The surface roughness of the collector and the polarization electrode makes it difficult to obtain tight contacts between the collector and the polarization and between the collector and the terminal plate. Loose contacts between the collector and the polarization and between the collector and the terminal plate result in large contact resistances between them. In order to reduce the contact resistance, it is necessary that the basic cell is pressured from opposite sides to the center so as to obtain the required tight contacts between them. Time-passing makes the contacts between them loose whereby the contact resistances and the equivalent series resistance increase. Further, a long-time use of the electric double layer capacitor under conditions of a high temperature and a high voltage load, a gas is likely to be generated in the interior of the capacitor whereby the polarization electrodes are peeled to increase the internal resistance.
It has been known that in order to reduce the contact resistance between the polarization electrode and the collector, the polarization electrode and the collector are bonded with each other by a conductive adhesive. In Japanese laid-open patent publication No. 3-28318, it is disclosed that a conductive adhesive material is applied on the collector and a polarization electrode is made into tightly contact with the conductive adhesive material applied on the collector by a heat press. In Japanese laid-open patent publication No. 7-86098, it is disclosed that a solution dispersed with an activated carbon is put into a collector in the shape of a container to cause carbonization to form a polarization electrode, before the collector and the polarization electrode are bonded with each other by a conductive adhesive. In Japanese laid-open patent publication No. 9- 148202, it is disclosed that a polyvinylidene chloride resin melt with a solvent is applied on the collector before a polarization electrode is placed on the polyvinylidene chloride resin applied on the collector, and then the polarization electrode is pressured toward the collector to heat up the same but at a temperature lower than a carbonization temperature whereby the polarization electrode and the collector are bonded with each other.
The above conventional methods by using the conductive adhesive are effective to reduce the contact resistance between the polarization electrode and the collector. Since, however, the conductive adhesive is not so low in the resistance, then it is difficult to considerably reduce the internal resistance of the electric double layer capacitor.
In Japanese laid-open patent publication No. 5-326326, it is disclosed that activated carbon is used for the polarization electrode whilst graphite powders are used for the collector, but the polarization electrode and the collector are formed or by sintering process in a single member which comprises two compositions, for example, activated carbon and graphite but which is compositionally graded so that, from one side to opposite side, a first compositional ratio of activated carbon is decreased from 100% to 0% continuously or discontinuously, whilst a second compositional ratio of graphite is increased from 0% to 100% continuously or discontinuously.
The above later conventional method is effective to reduce the internal resistance of the electric double layer capacitor. The electrode is, however, poor in softness such as rubber material. Also the polarization electrode part is brittle and breakable. Further, it is hard to adhere the collector to the gasket.
In the above circumstances, it had been required to develop a novel electric double layer capacitor free from the above problem.
Accordingly, it is an object of the present invention to provide a novel electric double layer capacitor free from the above problems.
It is a further object of the present invention to provide a novel electric double layer capacitor having thickness-reduced polarization electrode and collector.
it is a still further object of the present invention to provide a novel electric double layer capacitor having a reduced contact resistance.
It is yet a further object of the present invention to provide a novel electric double layer capacitor having a reduced equivalent series resistance.
It is another object of the present invention to provide a novel electric double layer capacitor which is highly reliable.
It is yet another object of the present invention to provide a novel electric double layer capacitor which has a long durability.
Also, it is another object of the present invention to provide a novel structure of polarization electrodes and collectors in an electric double layer capacitor free from the above problems.
It is a further object of the present invention to provide a novel structure of polarization electrodes and collectors in an electric double layer capacitor having thickness-reduced polarization electrode and collector.
It is a still further object of the present invention to provide a novel structure of polarization electrodes and collectors in an electric double layer capacitor having a reduced contact resistance.
It is yet a further object of the present invention to provide a novel structure of polarization electrodes and collectors in an electric double layer capacitor having a reduced equivalent series resistance.
It is another object of the present invention to provide a novel structure of polarization electrodes and collectors in an electric double layer capacitor which is highly reliable.
It is yet another object of the present invention to provide a novel structure of polarization electrodes and collectors in an electric double layer capacitor which has a long durability.
Also, it is another object of the present invention to provide a novel method of forming an electric double layer capacitor free from the above problems.
It is a further object of the present invention to provide a novel method of forming an electric double layer capacitor having thickness-reduced polarization electrode and collector.
It is a still further object of the present invention to provide a novel method of forming an electric double layer capacitor having a reduced contact resistance.
It is yet a further object of the present invention to provide a novel method of forming an electric double layer capacitor having a reduced equivalent series resistance.
It is another object of the present invention to provide a novel method of forming an electric double layer capacitor which is highly reliable.
It is yet another object of the present invention to provide a novel method of forming an electric double layer capacitor which has a long durability.
The present invention provides a unitary-formed electrode structure serving as both a collector and a polarization electrode. The structure comprises: a base material having an electrical conductivity for serving as the collector; and a polarization electrode material existing in at least a part of a surface region of the base material, so that at least the polarization electrode material is partially shown on the surface region of the base material.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.